1. Field of the Invention
The present invention relates to a method for super-sonically injecting oxygen into a melting furnace, especially a shaft furnace, in which the raw materials such as coke and scrap iron are loaded through the top and in which the combustion of the combustible materials is carried out by injecting air, generally preheated air, which reacts with the coke, the combustion having been initiated using preheated burners. These furnaces are especially cupola furnaces which comprise a toric annulus placed at the base of the cupola into which the blast air, preheated by heat exchange with the combustion gases, is injected through a multitude of nozzles connected to this toric annulus.
2. Related Art
To improve the operation of cupola furnaces, or to increase their production, it is known to inject oxygen by means of supersonic lances positioned in the center of each nozzle. One of the advantages of this technology is the penetration of oxygen into the center of the cupola due to the high oxygen injection velocity.
However, in the case of a low oxygen flow rate, the pressure of the oxygen in the lances decreases, this results in a decrease in the velocity of the oxygen injected into the cupola, (a velocity which becomes subsonic), the penetration of oxygen into the center of the cupola then being lower than at a high oxygen flow rate (with an upstream pressure of around 8 to 10×105 Pa in the case of the cupola).
In order to obtain a high oxygen velocity, the lances are generally sized for a working pressure of around 9×105 Pa (upstream of the convergent/divergent device that forms the supersonic injection nozzle positioned at the end of the lance). However, this pressure is only obtained at the nominal flow rate of the installation: it is only 4.5×105 Pa when operating at 60% of the nominal value.
To overcome this problem, it has already been proposed to make all the lances operate alternately either by alternating the “start” and “stop” regimes, or by alternating a “low flow rate” with a “high flow rate”. In both cases, the maximum flow rate is obtained at the working pressure of the lances. Thus, the lances are stopped from operating at low pressure which results in a low oxygen injection velocity.
These known techniques have, however, the following drawbacks:                complexity of implementation (installation cost);        reliability of solenoid valves subjected to a very large number of opening/closing cycles;        knowledge of the average flow rate consumed is difficult to establish, which does not facilitate comparison of these techniques relative to a stable flow rate; and        control of the overall flow rate is not continuous, but is in increments of flow rate.        
One alternative consists in operating an increasing number of lances, as a function of the flow rate in order to maintain the most stable pressure possible in the lances. Thus the low operating pressures are avoided when the oxygen flow rate is low.
However, there is generally an oxygen injection dissymmetry that is prejudicial to the correct operation of the cupola.
In all the case, the solutions described above require, in addition, the installation of an additional motor control.